1. Field of the Invention
Biaxially oriented polypropylene films are used in packaging and in industrial sectors, especially where there is a need for their advantageous properties, i.e. good optical properties, high mechanical strengths, good barrier action, in particular with respect to gases, good dimensional stability when heated and excellent layflat.
For most applications it is moreover desirable, e.g. for reasons of promotional effectiveness, to improve the optical properties of the polypropylene film, in particular its gloss and haze, while retaining its good processability. It is likewise desirable to improve the barrier properties of polypropylene films, for example to gain new applications.
2. Description of Related Arts
The prior art describes how the optical properties, in particular the gloss and haze, of biaxially oriented polypropylene film can be improved.
DE-A 43 06 155 describes a non-sealable, transparent, oriented polyolefin film having more than one layer, with a base layer and with at least one non-sealable outer layer made from polypropylene, where the outer layer comprises silicon particles which have had an organic aftertreatment and have an average particle diameter of from 2 to 6 .mu.m.
The haze and processing properties of the film are improved, but the text has no teaching, concerning improvement of the barrier properties of the film. Nor does it give any kind of indication as to how the topography of a film of this type should be adjusted for simultaneous improvement of gloss and oxygen barrier.
In packaging applications for food and drink, high barrier effectiveness against gases, water vapor and flavors is generally demanded (this having the same significance as low permeation). A well known process for producing packaging of this type consists in high-vacuum aluminum metallizing of the plastic films used for this purpose. Other well known processes consist in coating the films with oxidic materials (e.g. SiO.sub.x or Al.sub.x O.sub.y) or with water glass. Essentially, the coatings used are transparent.
The effectiveness of the barrier against the substances mentioned above depends essentially on the type of the polymers in the film and the quality of the barrier layers applied. Thus, very high barrier effectiveness against gases, such as oxygen and flavors, is achieved in metallized, biaxially oriented polyester films. An effective barrier against water vapor is best achieved in metallized, biaxially oriented polypropylene films. There are other applications in which both a very effective water vapor barrier and an acceptable oxygen barrier are desirable.
In the prior art, there is neither sufficient knowledge of the detailed basis for the barrier effect in metallized or oxidically coated polypropylene films nor of how this may be decisively improved. Variables which are clearly important are the substrate surface, the type of substrate polymer and its morphology. It is generally assumed that smooth substrate surfaces result in better barrier properties.
The dissertation by H. Utz (Technische Universitat Munchen, 1995: "Barriereeigenschaften Aluminiumbedampfter Kunststoffolien" [Barrier Properties of Aluminum-Metallized Plastic Films]) gives detailed results of investigations on the influence of substrate surface on barrier properties in various plastic films.